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special about being at rest relative to the earth. For instance, if a mattress
falls out of the back of a truck on the freeway, the reason it rapidly comes to
rest with respect to the planet is simply because of friction forces exerted by
the asphalt, which happens to be attached to the planet.
Galileo’s insights are summarized as follows:
The Principle of Inertia
No force is required to maintain motion with constant velocity
in a straight line, and absolute motion does not cause any
observable physical effects.
There are many examples of situations that seem to disprove the
principle of inertia, but these all result from forgetting that friction is a
force. For instance, it seems that a force is needed to keep a sailboat in
motion. If the wind stops, the sailboat stops too. But the wind’s force is not
the only force on the boat; there is also a frictional force from the water. If
the sailboat is cruising and the wind suddenly disappears, the backward
frictional force still exists, and since it is no longer being counteracted by
the wind’s forward force, the boat stops. To disprove the principle of inertia,
we would have to find an example where a moving object slowed down
even though no forces whatsoever were acting on it.
(a)(b)(c)
(d)(e)(f)
This Air Force doctor volunteered to ride a rocket sled as a medical experiment. The obvious effects on
his head and face are not because of the sled's speed but because of its rapid changes in speed: increasing
in (b) and (c), and decreasing in (e) and (f).In (d) his speed is greatest, but because his speed is not
increasing or decreasing very much at this moment, there is little effect on him.
Section 2.4The Principle of Inertia